Content Negotiation

Apache supports content negotiation as described in
the HTTP/1.1 specification. It can choose the best
representation of a resource based on the browser-supplied
preferences for media type, languages, character set and
encoding. It also implements a couple of features to give
more intelligent handling of requests from browsers that send
incomplete negotiation information.

Content negotiation is provided by the
mod_negotiation module, which is compiled in
by default.

A resource may be available in several different
representations. For example, it might be available in
different languages or different media types, or a combination.
One way of selecting the most appropriate choice is to give the
user an index page, and let them select. However it is often
possible for the server to choose automatically. This works
because browsers can send, as part of each request, information
about what representations they prefer. For example, a browser
could indicate that it would like to see information in French,
if possible, else English will do. Browsers indicate their
preferences by headers in the request. To request only French
representations, the browser would send

Accept-Language: fr

Note that this preference will only be applied when there is
a choice of representations and they vary by language.

As an example of a more complex request, this browser has
been configured to accept French and English, but prefer
French, and to accept various media types, preferring HTML over
plain text or other text types, and preferring GIF or JPEG over
other media types, but also allowing any other media type as a
last resort:

Apache supports 'server driven' content negotiation, as
defined in the HTTP/1.1 specification. It fully supports the
Accept, Accept-Language,
Accept-Charset andAccept-Encoding
request headers. Apache also supports 'transparent'
content negotiation, which is an experimental negotiation
protocol defined in RFC 2295 and RFC 2296. It does not offer
support for 'feature negotiation' as defined in these RFCs.

A resource is a conceptual entity
identified by a URI (RFC 2396). An HTTP server like Apache
provides access to representations of the
resource(s) within its namespace, with each representation in
the form of a sequence of bytes with a defined media type,
character set, encoding, etc. Each resource may be associated
with zero, one, or more than one representation at any given
time. If multiple representations are available, the resource
is referred to as negotiable and each of its
representations is termed a variant. The ways
in which the variants for a negotiable resource vary are called
the dimensions of negotiation.

A type map is a document which is associated with the handler
named type-map (or, for backwards-compatibility with
older Apache configurations, the MIME-typeapplication/x-type-map). Note that to use this
feature, you must have a handler set in the configuration that
defines a file suffix as type-map; this is best done
with

AddHandler type-map .var

in the server configuration file.

Type map files should have the same name as the resource
which they are describing, and have an entry for each available
variant; these entries consist of contiguous HTTP-format header
lines. Entries for different variants are separated by blank
lines. Blank lines are illegal within an entry. It is
conventional to begin a map file with an entry for the combined
entity as a whole (although this is not required, and if
present will be ignored). An example map file is shown below.
This file would be named foo.var, as it describes
a resource named foo.

Note also that a typemap file will take precedence over the
filename's extension, even when Multiviews is on. If the
variants have different source qualities, that may be indicated
by the "qs" parameter to the media type, as in this picture
(available as JPEG, GIF, or ASCII-art):

URI: foo

URI: foo.jpeg
Content-type: image/jpeg; qs=0.8

URI: foo.gif
Content-type: image/gif; qs=0.5

URI: foo.txt
Content-type: text/plain; qs=0.01

qs values can vary in the range 0.000 to 1.000. Note that
any variant with a qs value of 0.000 will never be chosen.
Variants with no 'qs' parameter value are given a qs factor of
1.0. The qs parameter indicates the relative 'quality' of this
variant compared to the other available variants, independent
of the client's capabilities. For example, a JPEG file is
usually of higher source quality than an ASCII file if it is
attempting to represent a photograph. However, if the resource
being represented is an original ASCII art, then an ASCII
representation would have a higher source quality than a JPEG
representation. A qs value is therefore specific to a given
variant depending on the nature of the resource it
represents.

MultiViews is a per-directory option, meaning it
can be set with an Options
directive within a <Directory>, <Location> or <Files> section in
httpd.conf, or (if AllowOverride is properly set) in
.htaccess files. Note that Options All
does not set MultiViews; you have to ask for it by
name.

The effect of MultiViews is as follows: if the
server receives a request for /some/dir/foo, if
/some/dir has MultiViews enabled, and
/some/dir/foo does not exist, then the
server reads the directory looking for files named foo.*, and
effectively fakes up a type map which names all those files,
assigning them the same media types and content-encodings it
would have if the client had asked for one of them by name. It
then chooses the best match to the client's requirements.

MultiViews may also apply to searches for the file
named by the DirectoryIndex directive, if the
server is trying to index a directory. If the configuration files
specify

DirectoryIndex index

then the server will arbitrate between index.html
and index.html3 if both are present. If neither
are present, and index.cgi is there, the server
will run it.

If one of the files found when reading the directory does not
have an extension recognized by mod_mime to designate
its Charset, Content-Type, Language, or Encoding, then the result
depends on the setting of the MultiViewsMatch directive. This
directive determines whether handlers, filters, and other
extension types can participate in MultiViews negotiation.

After Apache has obtained a list of the variants for a given
resource, either from a type-map file or from the filenames in
the directory, it invokes one of two methods to decide on the
'best' variant to return, if any. It is not necessary to know
any of the details of how negotiation actually takes place in
order to use Apache's content negotiation features. However the
rest of this document explains the methods used for those
interested.

There are two negotiation methods:

Server driven negotiation with the Apache
algorithm is used in the normal case. The Apache
algorithm is explained in more detail below. When this
algorithm is used, Apache can sometimes 'fiddle' the quality
factor of a particular dimension to achieve a better result.
The ways Apache can fiddle quality factors is explained in
more detail below.

Transparent content negotiation is used
when the browser specifically requests this through the
mechanism defined in RFC 2295. This negotiation method gives
the browser full control over deciding on the 'best' variant,
the result is therefore dependent on the specific algorithms
used by the browser. As part of the transparent negotiation
process, the browser can ask Apache to run the 'remote
variant selection algorithm' defined in RFC 2296.

Apache can use the following algorithm to select the 'best'
variant (if any) to return to the browser. This algorithm is
not further configurable. It operates as follows:

First, for each dimension of the negotiation, check the
appropriate Accept* header field and assign a
quality to each variant. If the Accept* header for
any dimension implies that this variant is not acceptable,
eliminate it. If no variants remain, go to step 4.

Select the 'best' variant by a process of elimination. Each
of the following tests is applied in order. Any variants
not selected at each test are eliminated. After each test,
if only one variant remains, select it as the best match
and proceed to step 3. If more than one variant remains,
move on to the next test.

Multiply the quality factor from the Accept
header with the quality-of-source factor for this variants
media type, and select the variants with the highest
value.

Select the variants with the highest language quality
factor.

Select the variants with the best language match,
using either the order of languages in the
Accept-Language header (if present), or else
the order of languages in the LanguagePriority
directive (if present).

Select the variants with the highest 'level' media
parameter (used to give the version of text/html media
types).

Select variants with the best charset media
parameters, as given on the Accept-Charset
header line. Charset ISO-8859-1 is acceptable unless
explicitly excluded. Variants with a text/*
media type but not explicitly associated with a particular
charset are assumed to be in ISO-8859-1.

Select those variants which have associated charset
media parameters that are not ISO-8859-1. If
there are no such variants, select all variants
instead.

Select the variants with the best encoding. If there
are variants with an encoding that is acceptable to the
user-agent, select only these variants. Otherwise if
there is a mix of encoded and non-encoded variants,
select only the unencoded variants. If either all
variants are encoded or all variants are not encoded,
select all variants.

Select the variants with the smallest content
length.

Select the first variant of those remaining. This
will be either the first listed in the type-map file, or
when variants are read from the directory, the one whose
file name comes first when sorted using ASCII code
order.

The algorithm has now selected one 'best' variant, so
return it as the response. The HTTP response header
Vary is set to indicate the dimensions of
negotiation (browsers and caches can use this information when
caching the resource). End.

To get here means no variant was selected (because none
are acceptable to the browser). Return a 406 status (meaning
"No acceptable representation") with a response body
consisting of an HTML document listing the available
variants. Also set the HTTP Vary header to
indicate the dimensions of variance.

Apache sometimes changes the quality values from what would
be expected by a strict interpretation of the Apache
negotiation algorithm above. This is to get a better result
from the algorithm for browsers which do not send full or
accurate information. Some of the most popular browsers send
Accept header information which would otherwise
result in the selection of the wrong variant in many cases. If a
browser sends full and correct information these fiddles will not
be applied.

The Accept: request header indicates preferences
for media types. It can also include 'wildcard' media types, such
as "image/*" or "*/*" where the * matches any string. So a request
including:

Accept: image/*, */*

would indicate that any type starting "image/" is acceptable,
as is any other type.
Some browsers routinely send wildcards in addition to explicit
types they can handle. For example:

Accept: text/html, text/plain, image/gif, image/jpeg, */*

The intention of this is to indicate that the explicitly listed
types are preferred, but if a different representation is
available, that is ok too. Using explicit quality values,
what the browser really wants is something like:

Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01

The explicit types have no quality factor, so they default to a
preference of 1.0 (the highest). The wildcard */* is given a
low preference of 0.01, so other types will only be returned if
no variant matches an explicitly listed type.

If the Accept: header contains no q
factors at all, Apache sets the q value of "*/*", if present, to
0.01 to emulate the desired behavior. It also sets the q value of
wildcards of the format "type/*" to 0.02 (so these are preferred
over matches against "*/*". If any media type on the
Accept: header contains a q factor, these special
values are not applied, so requests from browsers which
send the explicit information to start with work as expected.

New in Apache 2.0, some exceptions have been added to the
negotiation algorithm to allow graceful fallback when language
negotiation fails to find a match.

When a client requests a page on your server, but the server
cannot find a single page that matches the
Accept-language sent by
the browser, the server will return either a "No Acceptable
Variant" or "Multiple Choices" response to the client. To avoid
these error messages, it is possible to configure Apache to ignore
the Accept-language in these cases and provide a
document that does not explicitly match the client's request. The
ForceLanguagePriority
directive can be used to override one or both of these error
messages and substitute the servers judgement in the form of the
LanguagePriority
directive.

The server will also attempt to match language-subsets when no
other match can be found. For example, if a client requests
documents with the language en-GB for British
English, the server is not normally allowed by the HTTP/1.1
standard to match that against a document that is marked as simply
en. (Note that it is almost surely a configuration
error to include en-GB and not en in the
Accept-Language header, since it is very unlikely
that a reader understands British English, but doesn't understand
English in general. Unfortunately, many current clients have
default configurations that resemble this.) However, if no other
language match is possible and the server is about to return a "No
Acceptable Variants" error or fallback to the LanguagePriority, the server
will ignore the subset specification and match en-GB
against en documents. Implicitly, Apache will add
the parent language to the client's acceptable language list with
a very low quality value. But note that if the client requests
"en-GB; q=0.9, fr; q=0.8", and the server has documents
designated "en" and "fr", then the "fr" document will be returned.
This is necessary to maintain compliance with the HTTP/1.1
specification and to work effectively with properly configured
clients.

In order to support advanced techniques (such as cookies or
special URL-paths) to determine the user's preferred language,
since Apache 2.0.47 mod_negotiation recognizes
the environment variableprefer-language. If it exists and contains an
appropriate language tag, mod_negotiation will
try to select a matching variant. If there's no such variant,
the normal negotiation process applies.

Example

Apache extends the transparent content negotiation protocol (RFC
2295) as follows. A new {encoding ..} element is used in
variant lists to label variants which are available with a specific
content-encoding only. The implementation of the RVSA/1.0 algorithm
(RFC 2296) is extended to recognize encoded variants in the list, and
to use them as candidate variants whenever their encodings are
acceptable according to the Accept-Encoding request
header. The RVSA/1.0 implementation does not round computed quality
factors to 5 decimal places before choosing the best variant.

If you are using language negotiation you can choose between
different naming conventions, because files can have more than
one extension, and the order of the extensions is normally
irrelevant (see the mod_mime documentation
for details).

A typical file has a MIME-type extension (e.g.,
html), maybe an encoding extension (e.g.,
gz), and of course a language extension
(e.g., en) when we have different
language variants of this file.

Examples:

foo.en.html

foo.html.en

foo.en.html.gz

Here some more examples of filenames together with valid and
invalid hyperlinks:

Filename

Valid hyperlink

Invalid hyperlink

foo.html.en

foo
foo.html

-

foo.en.html

foo

foo.html

foo.html.en.gz

foo
foo.html

foo.gz
foo.html.gz

foo.en.html.gz

foo

foo.html
foo.html.gz
foo.gz

foo.gz.html.en

foo
foo.gz
foo.gz.html

foo.html

foo.html.gz.en

foo
foo.html
foo.html.gz

foo.gz

Looking at the table above, you will notice that it is always
possible to use the name without any extensions in a hyperlink
(e.g., foo). The advantage is that you
can hide the actual type of a document rsp. file and can change
it later, e.g., from html to
shtml or cgi without changing any
hyperlink references.

If you want to continue to use a MIME-type in your
hyperlinks (e.g.foo.html) the language
extension (including an encoding extension if there is one)
must be on the right hand side of the MIME-type extension
(e.g., foo.html.en).

When a cache stores a representation, it associates it with
the request URL. The next time that URL is requested, the cache
can use the stored representation. But, if the resource is
negotiable at the server, this might result in only the first
requested variant being cached and subsequent cache hits might
return the wrong response. To prevent this, Apache normally
marks all responses that are returned after content negotiation
as non-cacheable by HTTP/1.0 clients. Apache also supports the
HTTP/1.1 protocol features to allow caching of negotiated
responses.

For requests which come from a HTTP/1.0 compliant client
(either a browser or a cache), the directive CacheNegotiatedDocs can be
used to allow caching of responses which were subject to
negotiation. This directive can be given in the server config or
virtual host, and takes no arguments. It has no effect on requests
from HTTP/1.1 clients.

For HTTP/1.1 clients, Apache sends a Vary HTTP
response header to indicate the negotiation dimensions for the
response. Caches can use this information to determine whether a
subsequent request can be served from the local copy. To
encourage a cache to use the local copy regardless of the
negotiation dimensions, set the force-no-varyenvironment variable.

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